1. Field of the Invention
The present invention relates to a reusable externally applied ceramic thermal protection system (TPS) for use in extremely high temperature environments such as in commercial or portable ovens or on the exterior surfaces of aerospace vehicles which are subject to high temperature (up to 3200.degree. F.) and mechanical and thermal stresses during entry into planetary atmosphere. More particularly, the invention relates to a composite device which comprises a ceramic/ceramic upper shell which is attached to a ceramic variable density lower tile by means effective to survive the extreme thermal and stress environments to which the device is subjected. The composite device also may include one or more layers of rigid or flexible thermal insulation between the upper shell and the lower tile substrate. The upper shell effectively separates its primary function as a toughened outer load bearing skin having the proper emissivity properties, and is stress/strain independent of the lower tile.
Aerospace vehicles are subject to extremes of temperature (up to 3200.degree. F.), pressure and stress during ascent and re-entry. It is, therefore, customary to protect the surface of the vehicles with a heat shield designed to protect the vehicle's metallic substructure. The advent of the Space Shuttle created a need for improved thermal shielding devices and equipment. A number of new materials and devices were developed. Some patents of interest are cited below.
The presently used coating on the U.S. Space Shuttle reuseable surface insulation (RSI) is a reaction cured glaze (RCG). A major disadvantage of the reaction cured glass (RCG) glaze is its low resistance to physical impact. The technical data collected to date demonstrate that even minor impacts can cause cracks and pitting of the surface coating. These minor cracks are to be avoided because it allows for moisture penetration during rain, which increases vehicle weight and results in ice formation when exposed to a space environment. The latter causes stress and premature failure of the tile. Another major problem caused by the fragility of the coating, however, is that if the coating is breached, the underlying "soft" tile may be eroded by rain or ice at relatively low speeds, i.e. less than 300 miles/hr., thus requiring replacement.
2. Related Art
The existing references in this art recognize the disadvantages of present material and configurations. Reaction cured glass as described in U.S. Pat. No. 4,093,771 is a rigid borosilicate glass material having an emissivity control agent dispersed within it. RCG requires both a physical, thermal and chemical compatibility with the substrate to minimize and avoid thermal stress failures when it is subjected to thermal extremes. When the desired compatibility is obtained, the RCG and its substrate are highly resistant to thermal stress failures and are extremely thermally stable. When the physical and chemical property match is not obtained, during use both micro and macro cracks, and spallation or peeling of the coating from the substrate can occur. It is desirable to have a system that is tougher, and doesn't require a precise match with the substrate.
The use of composite and ceramic materials for aerospace vehicle applications have been under considerable investigation over the past 30 years. Some U.S. patents of interest regarding the materials of construction and fabrication of high temperature ceramics, fibers, composites and the like include, for example:
______________________________________ U.S. Pat. No. Inventor(s) General Subject Matter ______________________________________ 3,428,519 C. M. Zvanut Uniformly coating fibers with boron carbide. 3,603,260 C. C. Johnson Ablative heat shield. 3,702,261 F. Feakes, et al. Vacuum coating of plastic or metal substrates. 4,124,732 L. L. Leger Protection of space vehicle tiles using a strain isolation pad. 4,148,962 D. B. Leiser, et al. Refractory composite prepared from aluminoborosilicate and silica fibers. 4,226,914 Terver, et al. Powder composition of SiC and Si, plasma sprayed into a substrate. 4,287,259 Riley, et al. Uniformly coating fibers with boron carbide. ______________________________________
Additional U.S. patents of interest in the fabrication of compound fibers and structures include, for example: U.S. Pat. Nos. 3,709,706; 3,793,041; 3,793,041 and 4,125,406 (all of H. G. Sowman); 4,047,965 (of K. A. Karst); 4,283,376 (of S. Yajima et al.); and 4,314,827 (of M. A. Leitheiser et al.). None of these reference describe or suggest the present invention.
A patent which is incorporated herein by reference is U.S. Pat. No. 4,456,208, in which I. O. MacConochie, et al. disclose a metalic can shell structure as a thermal protection system. Both the upper shell and lower shell are metallic, usually of titanium and titanium alloys, nickel alloys, columbium or reinforced carbon composites. Although the metallic materials of construction are cited as being effective up to 2200.degree. F. and 3000.degree. F., the actual thermal tests disclosed for metallic structure devices have an upper temperature limit of 900.degree. F. Stress/strain independent ceramic/ceramic upper shells or ceramic variable density lower tiles are not disclosed or suggested. Further, it is generally regarded in the art that a ceramic composite created as an upper shell of the device may be too brittle to withstand the environmental thermal and stress conditions imposed by the launch and re-entry of an aerospace vehicle. Contact between lower surface 12 of the wedge 36 in FIG. 5 of this patent reference with upper surface 36 of the lower can is encouraged to strengthen the locking of the cans and to prohibit further downward movement of the upper shell. On the contrary, in the present invention, any such extra "support" will tend to cause additional unnecessary stress to the ceramic/ceramic upper shell and cause the upper shell to fail more easily. The device described in U.S. Pat. No. 4,456,208 is constructed of thin gauge metals having thermal expansion coefficients of 10.times.10.sup.-6 in/in .degree.F. When exposed to large temperature gradients, this type of construction may be subject to thermal stress failure. The present invention based on its physical properties and test data is not likely to be subject to such failure. Also, metals used on the outer surface have been found to act as catalysts for re-combination of oxygen and nitrogen atoms during atmospheric reentry. The ceramic composite of the present invention is somewhat noncatalytic and therefore has certain intrinsic advantages as a heat shield component.
The present invention describes a composite device comprising a ceramic/ceramic upper shell, a ceramic lower RSI tile system and the means effective to attach them. This invention is particularly useful because it provides (a) resistance to thermal stress, (b) temperature range capability, (c) durability in that it can be used and re-used, and (d) desired lightweight characteristics.